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Gearbox Overview

What Is A Gearbox

A gearbox uses mechanical advantage to increase output torque and reduce RPM. The motor's shaft is feed into the gearbox and through a series of internal gearing provides the torque and speed conversion. Our gearboxes are available in a variety of sizes and gear ratios to meet a wide range of torque requirements. The basic design is a spur gearbox with gear wheels in metal, plastic and combinations of the two materials. A particular feature is the availability of freewheels and slipping clutches.

Gearbox Basics

Saia motors gearboxes are available in a variety of sizes to meet a wide range of torque requirements. Ratios from 4 1/6 to 6.048.000 are available. The basic design is a spur gearbox with gear wheels in metal, plastics and combinations of the two materials. A particular feature is the availability of freewheels and slipping clutches.

The gearboxes are turned by the motor, energy flow is from input to output shaft. That means, they are not allowed to be driven by the output shaft (for instance turning manually).

This can lead to damage of some internal components!

Direction of rotation

Direction of rotation

As a function of the number of stages, the direction of rotation can be either clockwise or counter clockwise. The direction of rotation of motor gearbox units is generally specified by the gearbox output shaft (drive-side, see DIN EN60034-7, IEC 60050-411).

Ratio

Ratio

A gearbox is characterised by its gear ratio i or its time T. Gear ratio i is the ratio of input speed ne and output speed na. T is the time for one revolution of the output shaft.


Permissible force FA and FR at the output shaft

Permissible force FA and FR at the output shaft

Permissible force loads at the output shaft are:

  • Axial load FA, pulling or pushing in axial direction of the shaft
  • Radial load FR acting laterally on the shaft. The catalogue value is referred to a distance of 1 cm to the bearing

A gearbox is characterised by its gear ratio i or its time T. Gear ratio i is the ratio of input speed ne and output speed na. T is the time for one revolution of the output shaft.

Permissible Torque

The lifetime of a gearbox is determined by the load on the gear teeth and the number of revolutions of the gear wheels.

The maximum permissible torque Mn is defined by the load on the final stage of the gearbox and the stability of the housing.

Some gearboxes have lifetime graphs. It shows the relationship between ratio i and the associated torque for a fixed period of time, e.g. 1000 or 10000 hours. A conditional parameter is the input speed (equivalent to motor speed) corresponding to the total number of revolutions of all gear wheels. In the catalogue we show therefore two curves - for a motor having 250/300 rpm and 500/600 rpm.

For example: Max.output torque Mx1 is permissible at a ratio of ix1. With smaller ratios the max. permissible torque has to be reduced, because otherwise the first stages of the gearbox would be overloaded.

Permissible Torque

Additionally to the lifetime curve the motor torque Mm, multiplied by gear ratio and reduced by the gear efficiency, is shown (resulting in output torque Mi).

Example1: The application of motor 1 combined with a gearbox of ratio ix1 leads to an output torque Mx1 at point A. The gearbox can transmit this torque, meeting its lifetime.

If a ratio of i >ix1 is selected, actual torque would be M > Mx1. However lifetime cannot be guaranteed, as the operating point now lies above of the lifetime curve.

Example 2: Motor 1 with a ratio of ix2. Torque generated is Mx2. This is below of the lifetime curve. The transmission can operate for an extended period without difficulty.

Example3: Motor 2 and a ratio of ix3 give a torque of Mn. When using a ratio of i > ix3 - the gearbox cannot be loaded more than Mn.



Efficiency

Efficiency

The number of stages in the gearbox determines the efficiency. With high ratios of i this factor will decrease below 10%, as the graph below shows. (For UGO/UGP, UGR, see table in the chapter)

Saia motors Gearboxes Clutches

Gearbox types UGA, UGB and UGD can be fitted with freewheels or slipping clutches.

Freewheels transmit the max. torque M in the locked direction, <1 cNm in the opposite direction. One way slipping clutches behave similarly except that the slip torque has a higher value. Two way slipping clutches can only transmit a limited torque value in either direction lower than the slip torque.

Slipping clutches are used to: Protect the gearbox against torque overloads, or to adjust the load by turning from the load side (remember: turning the output shaft directly can otherwise damage the gearbox).

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Slipping clutchOne wayOne wayTwo way
Freewheel yes yes no
Torque clockwise full torque < slipping torque < slipping torque
Torque Anti clockwise < slipping torque full torque < slipping torque
Output shaft turning, clockwise slipping possible blocking slipping possible
Output shaft turning, anti clockwise blocking slipping possible slipping possible